A gas combustion type driving tool injects a combustible gas into a combustion chamber, hermetically sealed within a body, to stir a gas mixture of the combustible gas and air inside the combustion chamber, and combusts the stirred gas mixture inside the combustion chamber, thereby producing a high pressure combustion gas inside the combustion chamber. Further, this high pressure combustion gas acts on a hammering piston contained in a hammering cylinder, thus causing the hammering piston to be impulsively driven within the hammering cylinder. A driver connected to a lower face side of the hammering piston drives a nail, which has been supplied to a nose part located below the body, into a steel plate or concrete. By way of example, in such a combustion gas driven driving device, a container such as a gas bomb filled with a combustible gas is inserted into a tool. Furthermore, a battery serving as a power source for igniting a combustible gas is attached to the tool, thereby forming the tool as a portable tool. Therefore, an operation for driving a nail or a pin can be carried out without being restricted by a supply source of power such as electric power or compressed air.
Actually, the foregoing gas combustion type driving tool is provided with a feed mechanism for sequentially feeding, to a nose part, a connecting fastener housed in a magazine. As such a feed mechanism, there is generally used a feed piston/cylinder mechanism that has a feed piston slidably contained in a feed cylinder and provided with a feed claw engageable/disengageable to/from the connecting fastener housed in the magazine, and that reciprocates the feed piston in a nail feed direction in which the feed piston is fed toward the nose part and a retracting direction opposite to the feed direction.
In this case, as the feed piston of the feed piston/cylinder mechanism, there is known one in which the feed piston is reciprocated utilizing a spring and the pressure of a combustion gas inside the combustion chamber, the feed piston is constantly urged in the nail feed direction by the spring, and the feed piston is moved in the retracting direction against the spring by means of the pressure of the combustion gas at the time of hammering. In this technique, a gas pipe is located between the combustion chamber and the feed cylinder, and the combustion gas is sent to the feed cylinder via the gas pipe (JP-A-2006-315102).
However, since the volume of the combustion gas is significantly decreased upon cooling thereof, the pressure for holding the feed piston at a retracted position against the spring is also reduced. Further, the feed piston starts a fastener feed operation before predetermined timing, and the leading fastener inside the magazine is fed into the nose part so as to be brought into contact with and scraped against the driver, which is being returned, before the hammering piston is returned following the completion of a fastener driving operation. Hence, a problem that the hammering piston cannot be returned due to the resulting contact resistance might occur. Furthermore, there is also a possibility that a connecting material of the connecting fastener might be deformed, thus causing a feed failure.